@Toki
It' a known bug
See screen shots below
before
But I've got a SOLUTION just delete the directory "Your gp4 directory\GPSAVES\Carsets\1988\Data\Cars\Ferrari and it works now !!!
aflter delete
Do the same for Williams
delete directory: "Your gp4 directory\GPSAVES\Carsets\1988\Data\Cars\Williams
Do the same for Eurobrun
delete directory: "Your gp4 directory\GPSAVES\Carsets\1988\Data\Cars\Eurobrun
Do the same for AGS/OSELLA
delete directory: "Your gp4 directory\GPSAVES\Carsets\1988\Data\Cars\AGS-Osella
Do the same for MARCH
delete directory: "Your gp4 directory\GPSAVES\Carsets\1988\Data\Cars\March
And install 1988_Ferrari_Williams_Ags_Osella_Eurobrun.cmu that you can find
here.
And if you want March update
here